The present invention relates to the design of and the control of a pressing roll including hydrostatic pockets in which the rotary pressing casing of the roll is supported hydrostatically on an internal, non-rotary, longitudinal support.
The hydrostatic pocket containing pistons which are employed in these rolls are either round or elongate, as seen in a radial plan view.
Round pistons suffer from the disadvantage that under certain conditions, e.g. of breadth of the lands around the pockets and the gap width between the shoe and the roll casing, they involve a larger hydraulic periphery and hence a greater flow of hydraulic fluid and they furthermore require a greater driving power, using larger land areas.
For economy of operation, only longitudinally extending hydraulic pistons are acceptable, with shaped pressing shoes connected with the pistons. The shoes are either constituted by the pistons or are carried by them. For instance, there may be one, two, three or more such shoes distributed over the longitudinal length of the roll.
In German patent publication 2,502,161 Al, such a pressing shoe comprises a radially movable piston which is supported movably on the longitudinal support and is connected by a rocking joint with a saddle which is disposed radially outward of the piston and is adapted to the inner radius of the roll casing. The saddle contains hydrostatic pockets.
A broader and longer circumferentially middle pocket formed in the saddle is supplied with hydraulic fluid from the hydraulic pressure space underneath the piston and via holes through the rocking joint. The circumferentially external or flanking hydrostatic pockets are subdivided along the length of the saddle and receive hydraulic pressure fluid, i.e. oil, from the middle pocket via connecting holes between the middle and the external pockets, and that oil is drained off via external longitudinal lands into the essentially pressure free inner space of the roll.
Although a roll of this type has the advantage of a somewhat lower power dissipation for a given gap width and a given land breadth, it involves a relatively large gap height in order to prevent grating of the inner longitudinal lands on the roll casing caused by flexure of the saddle about the axis of rocking in the case of major supporting forces. However the consumption of hydraulic fluid increases with the cube of the gap width so that, for instance, in the case of a minimum gap width of 0.04 mm, a widening of the gap at the outer longitudinal lands by 0.02 mm will increase the fluid flow and hence the necessary pump power by a factor of i.e. 3.375.
A further disadvantage of the known system is that the rate at which oil is pumped into the hydrostatic pockets is proportional to the linear force of the pressing rolls and is inversely proportional to the viscosity of the oil. This means that the gap width between the lands of the saddle and the roll casing remains approximately constant independent of the linear force and the oil flow rate effective for the cooling of the roll, i.e. the removal of frictional heat generated during rotation of the casing by the lands, decreases at a low linear force and causes undesired, high temperatures, which, starting with the friction on the lands, leads to a temperature gradient in an inward direction and consequently to a heat induced bending of the saddle in the same direction as is caused by the flexure owing to supporting a force on the rocking joint. Therefore, during operation under either full load or even under partial load, with a low linear force, there will be widening of the gap along the outer lands which causes the above noted undesired effect for the required pump power.
The lower the temperature of the supplied hydraulic fluid or oil is, the greater is the heat induced curvature of the saddle, because the inner surface of the saddle is more strongly cooled. Owing to higher viscosity of the hydraulic fluid at a lower temperature, less hydraulic fluid will pass through the capillaries leading to all of the hydrostatic pockets. For this reason, there is less hydraulic fluid available for removal of the frictional heat which is produced in the gap between the saddle and the roll casing. The supply of cooler oil and the uneven thermal expansion cause the saddle to bend even more, and in an extreme case, the saddle may even grate against the inner lands on the roll casing, if counteraction is not taken by increasing the rate of flow of the hydraulic fluid.